A variety of techniques exist in the field of orthopaedic surgery for treating bone fractures. Many known techniques utilize bone screws and bone fixation plates. Typically, the plate is used to stabilize the site of a bone fracture, and one or more bone screws are inserted through apertures of the plate and threaded into the bone material so as to secure the plate to the bone material. It is also known that bone screw/plate systems can be improved by machining a thread onto the head of the bone screw, in addition to the thread normally machined onto the main shaft of the screw. In connection with the use of threaded-head screws, the apertures of the plate are threaded to matingly receive the threads of the screw head. Thus, as the screw is inserted into an aperture of the plate and threaded into the bone material, the head of the screw likewise is threaded into the aperture. As a result, the screw becomes rigidly affixed to the plate, in effect locking to the plate rather than simply bearing against the plate. Examples of threaded-head bone screws and threaded-aperture bone plates are disclosed in U.S. Pat. No. 5,709,686 to Talus et al.; U.S. Pat. No. 6,206,881 to Frigg et al.; and U.S. Pat. No. 6,306,140 to Siddiqui.
The use of threaded-head screws and threaded-aperture plates provides certain advantages. It is known that some types of small bone fragments tend to change position relative to the plate over time. This deleterious condition can result from the “toggling” of the screws affixed to the plate. However, when multiple screws are rigidly fixed to the plate by mating the respective threads of the screw heads with the threads of the corresponding plate apertures, the screws do not toggle in the plate. The locking action provided by the threaded-head screw in combination with the threaded-aperture plate prevents motion between the bone fragment and the plate as well as premature loosening of the screws.
Although the use of threaded-head screws has provided improvements in orthopaedic surgical techniques, there remains the disadvantage that currently available screw/plate systems are unidirectional. That is, the thread formed on the inside surface of the aperture of the plate is structurally fixed at a constant helical angle with respect to the central axis passing through the center point of the aperture. Hence, the head of a conventional threaded-head screw can only be rigidly affixed to the plate by mating its thread with that of the aperture, such that the bone screw is always inserted and threaded in one direction, e.g., perpendicularly or coaxially with respect to the plate.
It would therefore be advantageous to provide a screw/plate system that allows the surgeon to choose the angle at which the screw is inserted through, and rigidly affixed in, an aperture of the plate. Such an improvement would enable the surgeon to direct the bone screw toward bone fragments that are not situated directly beneath the aperture of the plate, and would also provide flexibility in the placement of the plate in relation to the bone fracture. The ability to choose the angle at which the screw is threaded into the bone material would allow the surgeon to better tailor the application of the screw/plate system to the specific nature of the bone fracture suffered by the individual orthopaedic patient, and additionally allow the surgeon to adjust his or her application strategy as necessary after the surgical site has been accessed but prior to insertion of the screw into the bone material. Additionally, in situations where a screw is intended for coaxial insertion into an aperture, the improvement would allow a secure fit between the screw and aperture even if the screw is unintentionally inserted in non-coaxial relation to the aperture.